July Southern Corn Rust Update

by T. R. Faske Extension Plant Pathologist - July 9, 2021

You can almost set your calendar by it, well maybe not, but southern rust typically comes into the state in July and this year was no exception. There were some reports of suspicious “southern rust like pustules” on corn in late June, but nothing confirmed until July. Southern rust was confirmed on July 6 (same as 2020) in Drew, Desha, Jefferson, Lonoke, Prairie, and Monroe counties. Disease incidence and severity was low (one leaf on two plants with 30-40 pustules/leaf: Fig. 1) on corn at blister to milk growth stage (R2-R3). Since then, other counties have been added to the Corn ipmPIPE website (Fig. 2). This update provides some guidelines the management of southern corn rust.

Southern rust pustules on leaf are rust-colored irregular dots grouped together

Figure 1. Southern rust pustules on lower and upper leaf surface from same leaf sampled in 2021 in Pulaski, Co.

See “Southern Rust” article by the Crop Protection Network for great information on southern rust identification and diagnosis

this map shows the distruction of southern rust. 9 Arkansas counties are positive and 9 are probable

Figure 2. Distribution of southern rust as of July 6, 2021. Red = positive, Yellow = probable, and green = scouted, but not found.

Dry conditions will suppress the spread of southern rust as free moisture (dew or light rain) is necessary for spore germination and infection. When conditions favor disease, symptoms appear about 3 to 6 days after infection and by 7 to 10 days the pustules rupture to release rust spores.

Conditions that favor disease:

warm/hot temperatures (morning low of 75°F and daytime high of 93°F + 4-6 hr of consecutive leaf wetness)
extended periods of light rain or heavy dew.

When these conditions are not met disease development will be much slower. In the current forecast, rainfall over the weekend will likely promote southern rust activity, so be aware of growth stages and if rust is present in the field.

Fungicide Timing for Southern Rust

Beginning Corn Growth Stage	Southern Rust in the Field	Weather Forecast Favors Southern Rust	Benefit from a Fungicide
VT- Vegetative Tassel	Yes	Yes	Yes
R1- Silk	Yes	Yes	Yes
R2- Blister	Yes	Yes	Yes
R3- Milk	Yes	Yes	Yes
R4- Dough	Yes	Yes	Unlikely
R5- Dent	Yes	Yes	No
R6- Maturity	Yes	Yes	No

Figure 3. Guide to protect corn yield potential with a fungicide when southern rust is first detected, and conditions favor disease development.

Fungicides are effective at protecting corn yield potential, but fungicide use should be made on a field-by-field basis. The “Fungicide Timing for Southern Rust” table” is a guideline on the benefit of a fungicide to protect yield potential at various corn growth stages with the assumption that southern rust is present (first detection) in the field and conditions favor disease development (Figure 2). See MP 154 for fungicides efficacy tables to control southern rust in Arkansas.

Please contact tfaske@uada.edu for any comments or questions.